Solid phase extraction is a chromatographic technique of frequent use in the preparation of samples for quantitative analysis, for example, via high performance liquid chromatography (HPLC) or gas chromatography (GC) (McDonald and Bouvier, eds. Solid Phase Extraction Applications Guide and Bibliography, sixth edition, Milford, Mass.: Waters (1995)). Solid phase extraction can be used to separate a component of interest in a complex solution from potentially interfering matrix elements and to concentrate the analyte to levels amenable to detection and measurement. Thus, solid phase extraction is of use in the analysis of environmental samples, where, for example, various soluble components of soils may interfere with the analysis of trace organic materials. Solid phase extraction is also of importance in the analysis of pharmaceutical agents or metabolites in blood plasma, which requires the prior removal of plasma proteins and other matrix constituents which may interfere with the analysis.
Solid phase extraction of an aqueous solution is typically performed by passing the solution through a single-use cartridge containing a chromatographic sorbent. The most commonly used sorbents consist of porous silica particles that have been functionalized on their surface with hydrophobic octyl (C8) and octadecyl (C18) functional groups. Prior to use, such sorbents must be wetted with a water-miscible polar organic solvent to solvate the alkyl chains. This increases the contact of these chains with the aqueous phase, increasing the sorbent surface area available to solutes and, therefore, retention of solutes. Such sorbents which are not pre-wetted or have dried out display poor solute retention, and, thus, inadequate separation of solution components.
The requirement that the sorbent remain wetted during the extraction procedure complicates solid phase extractions and substantially slows sample analysis. For example, solid phase extraction cartridges, in general, have differing flow rates and must be monitored individually to prevent drying out when used on a vacuum manifold, the current state of the art for processing multiple samples. This further complicates the development of instruments for automated solid phase extraction, which often incorporate elaborate safeguards to prevent drying out of the sorbent.
Thus, there is need for a solid phase extraction method which utilizes a sorbent that does not require wetting with an organic solvent or that stays wetted even if the bulk of the wetting solvent is removed during use on a vacuum manifold. Such a method would enable more rapid sample preparation for quantitative analysis, particularly for multiple samples, and allow the development of less expensive and simpler methods for automated solid phase extraction.
The present invention relates to a method for removing an organic solute from a solution. The method comprises contacting the solution with a water-wettable polymer formed by copolymerizing one or more hydrophobic monomers and one or more hydrophilic monomers, whereby the solute is adsorbed onto the polymer. The solution can comprise a polar solvent such as a polar organic solvent, a water/organic mixture or, preferably, water or an aqueous solution, such as an aqueous buffer, acid, base or salt solution.
The hydrophobic monomer can comprise a hydrophobic moiety. Suitable hydrophobic moieties include, but are not limited to phenyl, phenylene and C2-C18-alkyl groups. Suitable hydrophobic monomers include divinylbenzene and styrene.
The hydrophilic monomer can comprise a hydrophilic moiety. In one embodiment the hydrophilic moiety is a saturated, unsaturated or aromatic heterocyclic group, such as a pyrrolidonyl group or a pyridyl group. In another embodiment, the hydrophilic moiety is an ether group. Suitable hydrophilic monomers are, for example, N-vinylpyrrolidone, 2-vinylpyridine, 3-vinylpyridine, 4-vinylpyridine and ethylene oxide.
In one embodiment of the method, the polymer is a poly(divinylbenzene-co-N-vinylpyrrolidone) copolymer which comprises greater than about 12 mole percent N-vinylpyrrolidone. In a preferred embodiment, the copolymer comprises from about 15 mole percent to about 30 mole percent N-vinylpyrrolidone.
The present invention further includes a method for forming a solution, containing a solute, which is suitable for quantitative analysis. In one embodiment, the method comprises contacting a first solution including the solute with a water-wettable polymer formed by copolymerizing at least one hydrophobic monomer and at least one hydrophilic monomer, whereby the solute is adsorbed onto the polymer. This is followed by washing the polymer with a suitable solvent or mixture of solvents, so that the solute is desorbed from the polymer, thereby forming a second solution including the solute. This second solution is suitable for quantitative analysis.
In another embodiment, the invention provides a method for forming a solution comprising a polar organic solute which is suitable for quantitative analysis. The method comprises contacting a solution which includes the polar organic solute and at least one additional solute of lesser polarity with a water-wettable polymer formed by copolymerizing at least one hydrophobic monomer and at least one hydrophilic monomer, whereby the additional solute is adsorbed onto the polymer and the polar solute remains in the aqueous phase. The resulting aqueous phase is, thus, a solution of the polar organic solute which is suitable for quantitative analysis.
The present invention further includes a solid phase extraction cartridge comprising an open-ended container and a polymer packed within the container. The solid phase extraction cartridge can, optionally, further comprise a porous retaining means, such as a frit. The polymer is formed by copolymerizing at least one hydrophobic monomer and at least one hydrophilic monomer. Suitable polymers include poly(divinylbenzene-co-N-vinylpyrrolidone) copolymers which comprise about 12 mole percent or more, preferably from about 15 mole percent to about 30 mole percent, N-vinylpyrrolidone. The solid phase extraction cartridge preferably comprises from about 0.025 g to about 1 g of the polymer.
The present invention enables the solid phase extraction of one or more solutes from an aqueous solution, without prior wetting of the sorbent with an organic solvent. The method is versatile with respect to solute identity, resulting in extraction of a broad range of solutes of varying polarity. A particular advantage of the method is that the sorbent can dry out during the extraction procedure without diminishing the ability of the sorbent polymer to retain solutes. Thus, the present invention provides a simpler method for the preparation of analytical samples, decreasing sample preparation time and increasing sample throughput. The present method is, thus, also more amenable to automation than currently used methods.